Source file src/pkg/crypto/rc4/rc4.go

     1	// Copyright 2009 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	// Package rc4 implements RC4 encryption, as defined in Bruce Schneier's
     6	// Applied Cryptography.
     7	package rc4
     8	
     9	// BUG(agl): RC4 is in common use but has design weaknesses that make
    10	// it a poor choice for new protocols.
    11	
    12	import "strconv"
    13	
    14	// A Cipher is an instance of RC4 using a particular key.
    15	type Cipher struct {
    16		s    [256]byte
    17		i, j uint8
    18	}
    19	
    20	type KeySizeError int
    21	
    22	func (k KeySizeError) Error() string {
    23		return "crypto/rc4: invalid key size " + strconv.Itoa(int(k))
    24	}
    25	
    26	// NewCipher creates and returns a new Cipher.  The key argument should be the
    27	// RC4 key, at least 1 byte and at most 256 bytes.
    28	func NewCipher(key []byte) (*Cipher, error) {
    29		k := len(key)
    30		if k < 1 || k > 256 {
    31			return nil, KeySizeError(k)
    32		}
    33		var c Cipher
    34		for i := 0; i < 256; i++ {
    35			c.s[i] = uint8(i)
    36		}
    37		var j uint8 = 0
    38		for i := 0; i < 256; i++ {
    39			j += c.s[i] + key[i%k]
    40			c.s[i], c.s[j] = c.s[j], c.s[i]
    41		}
    42		return &c, nil
    43	}
    44	
    45	// XORKeyStream sets dst to the result of XORing src with the key stream.
    46	// Dst and src may be the same slice but otherwise should not overlap.
    47	func (c *Cipher) XORKeyStream(dst, src []byte) {
    48		for i := range src {
    49			c.i += 1
    50			c.j += c.s[c.i]
    51			c.s[c.i], c.s[c.j] = c.s[c.j], c.s[c.i]
    52			dst[i] = src[i] ^ c.s[c.s[c.i]+c.s[c.j]]
    53		}
    54	}
    55	
    56	// Reset zeros the key data so that it will no longer appear in the
    57	// process's memory.
    58	func (c *Cipher) Reset() {
    59		for i := range c.s {
    60			c.s[i] = 0
    61		}
    62		c.i, c.j = 0, 0
    63	}